This invention relates to digital binary selective calling systems having group call capability and more particularly to such capability in asynchronous detectors, as in pagers, utilizing two-word, binary addresses.
In the field of paging and two-way data communications devices, there are two basic methods of address coding. The first type uses a sequence of tones modulated on a carrier frequency. Each unit, upon detecting the first tone of its unique address, listens for the second tone. If that tone is detected within a specified time interval, the unit listens for the third tone of its code, etc. Upon detection of the complete sequence of tones, an alert signal is provided, typically an audible tone. Group call capability can be added to a tone-addressed unit with additional circuitry, typically available as an optional module. For interface with telephone circuitry, however, digital binary addressing has certain advantages. Thus, units have been developed which use digital address codes, such as two "words", each made up of a number of bits of binary information. Such codes can provide for many more unique "addresses" on a given carrier frequency than tone codes can.
Digitally addressed units usually require that synchronizing information be provided in advance of the address code, to establish the beginning of the first word in order that the address may be accurately detected. A system providing a means of detecting an asynchronous address within a train of binary signals is disclosed in U.S. Pat. No. 3,801,956 and assigned to the same assignee as the present invention. In this system, the received binary word is entered into a sample register and compared bit by bit to an address word stored in a storage register. If a sufficient number of correlations are detected and counted, an alert signal is provided. Since the incoming binary word is cycled through a comparator, certain constraints are put on the words which can be used in a given system. In another U.S. Pat. No. 3,855,576, also assigned to the same assignee as the present invention, the above asynchronous detection method is applied to word one of a two word code sequence. When a paging receiver detects word one asynchronously, a counter is enabled and counts for a period long enough for a second received word to be entered into the sample register. Word two of the page code address is put into the storage register and compared with the second received word during a brief window at the end of the counting period. If a predetermined number of correlations is counted, an alert signal is then provided. In the first-noted patent, when using one 23 bit word, only 178 different words would be available in a single paging system. The system of the last-noted patent also allows for over 100 different first words, but since the word one detect provides synchronization for the detection of word two, over 4,000 different second words are possible. Thus, over 400,000 distinct two word combinations are available in a single system. However, in this system simultaneous addressing of a number of units is possible only by assigning them the same code address, making individual addressing possible only with two separate address codes for each unit.